The present invention is directed to a three-phase induction motor which is arranged for operation from a single phase power supply.
A three-phase motor is desired especially at higher horsepower ranges, for example, above 1 hp, because of its inherent efficiency over a single-phase type motor. These types of motors in effect generate a second phase (especially for starting) by a second winding in series with a capacitor. Such a capacitor run motor is usually more expensive and has lower efficiency than a corresponding equal power three-phase motor.
However, a three-phase motor can be operated from a single phase power supply by two well-known techniques. The first is a static phase converter and the second a rotary phase converter. Both are manufactured by Ronk Electrical Industries, Inc., of Nokomis, Ill. Both are discussed in a paper entitled, "Current Techniques in Phase Conversion Systems," by Claude M. Hertz, IEEE Conference Paper No. 78-83, IEEE Rural Electric Power Conference, Minneapolis, Minn., May 1-2, 1978.
More specifically, the static phase converter has an autotransformer and a series capacitor which goes to the third terminal of the three-phase motor. The tap off the autotransformer is chosen by the user as a function of the full load power facto of the user's motor. The capacitor size is selected by the manufacturer as a function of the motor current or motor horsepower.
There are two major disadvantages to the static converter:
1. A heavy expensive transformer is required; and PA1 2. The motor currents and voltages are balanced only at full load on the shaft.
To quote from the article by Mr. Hertz:
"The major limitations are obvious from these characteristics; wide variations from the given load point should be avoided and breakdown torque is limited because of the reduced manufactured current (I.sub.A) at overload. PA0 The static phase converter is generally suited for single motor applications where the motor load remains relatively constant . . . It does not provide very good motor breakdown torque, does not operate well under extreme motor load variations and is usually more expensive per horsepower than other phase converter types."
The rotary phase converter, while well suited for multiple motor applications, has disadvantages in that "its idle losses are high and typically has limited starting torque. Motor current unbalance is usually high, with an associated decrease in motor life to be expected . . . ".